The present invention relates to an arrangement for and a method of treating pulp, and a method of modernizing a pulp tower. The arrangement and method relate to treating pulp in connection with high-consistency pulp towers, and especially to improvements in discharging pulp therefrom. The invention also relates to modernizations of pulp towers. High-consistency pulp towers are used in the wood processing industry, for instance, for bleaching and/or storage of high-consistency pulp.
According to prior art, pulp has to be discharged in a diluted form from high-consistency pulp towers. This is because high-consistency pulp cannot be pumped with, for example, a centrifugal pump, which, however, in recent arrangements is practically the only way of conveying pulp from one process stage to another. Therefore, high-consistency pulp (having most commonly a consistency of 20 to 35%) is diluted to at least a medium consistency (of about 10 to 15%) in the bottom part of the pulp tower. This makes the pulp pumpable with a so-called fluidizing centrifugal pump. Preferably, pulp is diluted to a consistency of about 3 to 5%, whereby it will be pumpable with a conventional centrifugal pump. Dilution is effected by introducing either clean water or filtrate from a suitable process stage into the bottom part of the tower and mixing it with the pulp by agitators arranged for that purpose in the bottom part, i.e. a so-called dilution zone of the tower.
Depending on whether high-consistency pulp towers are used for bleaching or storage, the constructions and appearances of their bottom parts are much different from each other for a number of reasons. Specific to all types of towers is, however, that even dilution is almost unattainable. The reason for this is that high-consistency pulp as well as medium-consistency pulp flows downwards in the tower unevenly. This again is caused by friction between the pulp and the tower wall, which retards the pulp flow so much that between the zone of the diluted pulp in the bottom part and the undiluted pulp in the upper part of the tower there will be formed an arch, which, after having expanded enough, will collapse down to the bottom part of the tower. Since dilution liquid is introduced as an even flow into the tower, the pulp to be discharged from the tower is continuously diluted during arching and, immediately after the arch has collapsed, the consistency will increase to a maximum, whereby the required pulp consistency will remain somewhere between the maximum and the minimum values. In one high-consistency pulp tower the discharge consistency has been established to range from 3.2 to 6.1%. As the pulp is in most cases conveyed from the high-consistency pulp tower to some other process stage, whereby chemicals are mixed with it in pumping or soon thereafter, it is easy to understand that the chemicals dosage per pulp unit cannot be even when the consistency ranges so drastically. Another problem resulting from the collapse of the high-consistency pulp down to the bottom part of the tower may also be difficult, namely it is quite possible that the agitator is damaged by the great volume of pulp falling onto it. In the worst case, the entire process has to be stopped for the repairs of the agitator.
Another way of arranging even downward flow of pulp is described below. In the smallest towers having a diameter of about 3.5 to 7.0 m, the bottom part may be either straight cylindrical or first somewhat narrowing and below that cylindrical. In bigger towers having a diameter typically larger than 5.0 m, a so-called bottom pillar is disposed at the center of the tower bottom. The purpose of the bottom pillar is to uphold pulp above the bottom part and to divide the bottom part into an annular mixing zone. Thus, for example, the maximum diameter of the collapsing pulp arch may only be as long as the tower radius, whereas in the towers with no bottom pillar it may be equal to tower diameter. The shape of the prior art bottom pillars may be either an evenly converging cone, a cylindrical pillar, or a cylindrical pillar the upper end whereof is arranged with an upwardly converging cone. In all those towers, which are provided with a bottom pillar, the dilution agitator/dilution agitators are disposed on the sides of the bottom pillar so that they direct the flow to circulate along the annular mixing zone. The bottom pillars are of solid construction and when disposed on the tower bottom they are merely supported by the tower bottom or the foundation therebelow, in any case by the very point, which would also otherwise carry the weight of the pulp in the tower.
However, it has been shown in practice that neither more conical or cylindrical bottom pillars nor combinations thereof can eliminate the unevenness of the pulp discharge consistency. As already discussed above the discharge consistency may fluctuate from 3.2 to 6.1% when a bottom pillar according to prior art is used. Correspondingly, also the volume flow of the pulp being discharged fluctuates from 210 to 240 m3/h because the centrifugal pump is not at all insensitive to remarkable changes in the consistency.
At least some of the above-mentioned disadvantages have been overcome with a pulp tower according to U.S. Pat. No. 5,711,600, which discusses an improved high-consistency pulp tower, the bottom part of which has been provided with a bottom pillar of a new shape. The bottom pillar is preferably cylindrical, although other cross-sectional shapes are also applicable. The upper end of the bottom pillar has, however, been reshaped in comparison with prior art constructions. It is essential to the upper end of the pillar that the diameter of a parting member disposed therein is at least in one point larger than the diameter of the lower part of the pillar. In other words, it is a feature of the parting member that in the area of the parting member, the cross-section between the parting member and the wall of the tower is smaller than in the bottom area of the pillar. In accordance with an embodiment of the parting member it is formed of a first section, the diameter of which widens conically upwards, and of a second section, the diameter of which converges conically upwards. In other words, at the contact point between the first and second sections the diameter of the parting member is at its largest, whereby a throttle is formed between parting member and tower wall. The purpose of this throttle is to even the downward flow of the high-consistency pulp.
It has to be noted, however, that the term “conical” has been used above and will be also used further below to specify a piece widening, or correspondingly converging, in some direction. So, in practice, the conical parting member is replaceable with, for example, a quadrangular, a pentangular, or a hexagonal jacket. Correspondingly, the term “diameter” may as well refer to a diameter of an imaginary circle calculated on the basis of the area defined by the above-mentioned polygonal jackets.
However, in experimenting with this new bottom pillar, and its parting member, it has been learned that though the bottom pillar works in a much more reliable manner than the older prior art bottom pillars, the operation thereof can still be improved. For instance, it has been learned that the dilution zone at the bottom part of the tower tends to rise to the level of the parting member or even above it. Also, in some specific cases it has been learned that the dilution arranged by means of, for instance, diluting agitators at the bottom part of the tower is not sufficient, and it should be improved.
The above problems occur especially when the consistency of the fiber suspension in the storage, i.e. the upper, part of the tower is high, and the consistency of the suspension to be discharged from the tower is rather low. This requires that a huge amount of dilution liquid has to be introduced into the pulp. The following example describes a mill-scale case where the pulp storage tower contains fiber suspension in a 30% consistency, and the treatment apparatus after the tower requires 139 l/second of pulp in the consistency of 4%. This means that about 120 l/sec. of dilution liquid has to be provided in the tower. Since normal practice is to add some 30 l/sec. in the outlet pipe where the consistency is adjusted to match exactly the required consistency, the amount of dilution liquid to be added in the dilution, i.e. the bottom, part of the tower is about 90 l/sec. The practice has shown that the diluting agitators of a reasonable size can feed about 20 l/second dilution liquid. Otherwise, the size of the agitators would have to be increased, which is not practical, as it would result in increasing power consumption and increasing height of the dilution part due to increased length of the agitator blades. Thus the only option would be to add the number of agitators to five, which is more than would be needed for proper agitation of pulp.
However, both adding the number of agitators and increasing the agitator size would increase the investment costs and energy consumption. It would also lead to the weakening of the tower structure as either the size or the number of agitator openings in the tower wall would increase. This, in turn, would result in the use of an increased wall thickness of the towers, which leads again to increased investment costs.